The cost base for an offshore windfarm development includes multiple aspects. The fabrication and installation of foundations (structure system including piles) can account for 25% of the overall cost and can contribute significantly to cost increase from supply chain constraints and volatility in the price of material. Therefore, reducing the cost of foundations could have a significant impact on the overall capital cost of a project.

The Transition Piece [TP] is the component between the lattice jacket and the turbine tower and is the key component in the loads transfer between the turbine and substructure. It is relatively heavy and more complex than other parts of the jacket structure and often incurs a large percentage of the foundation cost. Therefore, its optimization will provide significant saving in the overall foundation system.

This paper will address this particular aspect of the jacket design, showing how it enhances economic competitiveness whilst at the same time dramatically improving structural integrity by reducing stress concentrations by a factor of 5 in key areas resulting in an increase in fatigue life of 125 times.

Method

The paper focuses on the need to keep costs down while deploying larger turbines in deeper water. It discusses the structural jacket foundation system for offshore wind turbine and looks in to several components: tower, transition piece, jacket substructure, their function and design requirements. The paper will look in more detail at the evolution of the design solutions for the transition piece, considering their design strengths and weaknesses before focusing on the Plated Transition Piece (PTP). By focusing on the unique features of the PTP the paper will show how the design innovation within the PTP results in a transition piece which is simple to fabricate, light in weight and flexible in design.

Results

The paper will show with reference to stress and other analysis how the Plated Transition Piece performs relative to other transition piece designs. It will describe how this innovate design results in a substantial reduction in stress concentrations that allows the mass of steel required to accommodate the loads to be reduced without resorting to the use of castings or forgings and so keep costs low.

Conclusions

The design of TP in a jacket structure for offshore wind foundation has been challenging, especially for its fatigue resistance. The design makes use of simple plates and an innovative connection, and is henceforth referred as Plated Transition Piece or PTP.

This PTP design therefore has achieved the following:

· Design simplification with the ability to enhance local fatigue strength without changing global parameters

· Easy fabrication

· Relatively light weight

· High fatigue strength from reduced stress due to a combination of:

o Simple load transfer

o stress relieving wing detail

· Reduced CAPEX cost

· Easy maintenance and repair leading to reduced OPEX cost

The new design provides a new direction for load transfer and fatigue detailing for transition piece design.

Objectives

1. Recognize that it is possible to introduced cost saving and 'innovation' into proven systems.

2. See how this approach provides a reduced risk profile, a key factor in the development of a nascent industry

3. See how improved optimized design can also bring savings in terms of fabrication, allowing access to less specialized local manufacturing services, easy repair and reduced O&M costs